The reliability of the airtight sealed section is improved by providing a
metallic or ceramic insertion member at a portion positioned between an
electricity introducing member and a narrow tube in the airtight sealed
section. The difference between the inner diameter of the narrow tube and
the outer diameter of the insertion member is made 0.02 to 0.6 mm. The
electricity introducing member is constructed by a halogen-resistant first
member and a second member whose coefficient of linear expansion is
similar to that of the narrow tube, and the junction of the first member
and second member is covered with a halogen-resistant glass sealant. The
difference between the insertion length of the second member into the
narrow tube and the flow-in length of the glass sealant into the narrow
tube is made 1 mm or more.
